MATLAB: Are there any built-in way to calculate inversions

inversion

In Mathematica, there is a built-in function

Inversions

which counts the number of inversions in permutation.

See the document for details at https://reference.wolfram.com/language/Combinatorica/ref/Inversions.html

Is there a similar function in Matlab?

Thanks.

Best Answer

Hi Ivor,

I do not know of one, but here is one way to accomplish it

p = [3 6 1 4 5 2];   % for example
inversions = 0;
for k = length(p):-1:2
  f = find(k==p);
  inversions = inversions + k -f;
  p(f) = [];
end
inversions                % the result

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MATLAB: Convolution built-in function

What John and Stephen said. What you appear to want (and wanted originally) is just

synthetic=conv(RC,amp);

which would be the convolution of RC and the amp kernel.

Prior to HG2 (handle graphics 2), graphics and other handles were just numeric values (doubles) so arithmetic operations on them were not illegal and so you didn't get an error in earlier release but the answer was nonsensical. Now, handles are a new class and so such erroneous operations can be flagged as errors.

You will want to revisit any calculations done earlier as they are in error; the result would have been just a constant times the vector.

R2012b
>> h=plot(rand(4,1))
h =
174.0016
>> whos h
Name      Size            Bytes  Class     Attributes
h         1x1                 8  double              
>> conv(h,1:4)
ans =
174.0016  348.0032  522.0048  696.0063
>> [1:4]*h
ans =
174.0016  348.0032  522.0048  696.0063
>> 
R2014b
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>> h
h = 
Line with properties:
 ...
>> whos h
Name      Size            Bytes  Class                                   Attributes
h         1x1                60  matlab.graphics.chart.primitive.Line              
>> 
>> double(h)
ans =
 1.2207e-04
>>

So, you see that prior to HG2 plot returned a double that was a floating point value; the specific value is dependent upon each invocation, but 175+/- is typical. OTOH, with HG2 as R2014b shows, the handle is a class variable and you can't see the numeric value unless you cast it to double and you see the numeric value is grossly different.

Either, way, using the line handle in conv makes no sense computationally, whatsoever.

MATLAB: Shadowing built-in functions

I am not using packages either (yet), but I would love to have an alternate version of ADDPATH which would mount folders as packages (basically just generating a name space). It would also be interesting to be able to set MATLAB in a mode where its toolboxes must be "mounted" as packages. E.g.

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 which -all regress
 'regress' not found.

Best Answer

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